iomap

iomap grew out of need to provide modern block mapping abstraction for filesystems three different IO access methods:

• Direct IO
• Buffered IO
• DAX

Block mapping provides a mapping between data cached in memory, in the page cache, and the location on persistent storage where that data lives. LWN has an incredible review of the old buffer-heads block-mapping and why they are inefficient, since the the inception of Linux. Since buffer-heads work on a 512-byte block based paradigm, it creates an overhead for modern storage media which no longer necessarily works only on 512-blocks. This document strives to provide a template for LSFMM for what will hopefully eventually become upstream Linux kernel documentation for iomap and guidance for developers on converting a filesystem over from buffer-heads to iomap.

A modern block abstraction

Instead of assuming a granularity of storage media 512-blocks at time, iomap allows filesystems to query storage media for data using block ranges. Since block mapping are provided for a block ranges for cache data in memory, in the page cache, naturally this implies operations on block ranges will also deal with multipage operations in the page cache. Folios are used to help provide multipage operations in memory.

struct iomap_ops

A filesystem is encouraged to provide struct iomap_ops for beginning an IO operation and ending an IO operation on a block range, and so the struct iomap_ops data structure has iomap_begin() and iomap_end() callbacks. You would call iomap with a specialized iomap operation depending on its filesystem or page cache interactions. For example iomap_dio_rw() would be used for Direct IO. So for example, on your fileystems's respective struct file_operations.write_iter() you'd eventually call iomap_dio_rw(..., &filesystem_direct_write_iomap_ops, &your_filesystem_dio_write_ops…) when dealing with Direct IO on the write_iter(). For buffered IO you'd use iomap_file_buffered_write(..., &your_filesystem_buffered_write_iomap_ops) on the same struct file_operations.write_iter(). But that is not the only situation in which a filesystem would deal with buffered writes, you could also use buffered writes when a filesystem does struct file_operations.fallocate() and for this case there is a special respective iomap_zero_range(..., &your_filesystem_buffered_write_iomap_ops). However struct file_operations.fallocate() also supports truncation, and for that you'd use iomap_truncate_page(..., &your_filesystem_buffered_write_write_iomap_ops). We'll elaborate on these more below.

Experience in adopting iomap on XFS has has shown that the filesystem implementation of these operations can be simplified considerably if one struct iomap_ops is provided per major filesystem IO operation:

• read
• direct writes
• DAX writes
• buffered writes
• xattr - FIEMAP_FLAG_XATTR
• seek

In terms of simplicity, you may convert a filesystem in this order instead:

For example:

• struct iomap_ops xfs_read_iomap_ops` iomap: lift the xfs writeback code to iomap

• struct iomap_ops xfs_direct_write_iomap_ops

• struct iomap_ops xfs_dax_write_iomap_ops

• struct iomap_ops xfs_buffered_write_iomap_ops - xfs: split out a new set of read-only iomap ops

• struct iomap_ops xfs_xattr_iomap_ops - xfs: fix SEEK_DATA for speculative COW fork preallocation

• struct iomap_ops xfs_seek_iomap_ops - iomap: move the iomap_dio_rw ->end_io callback into a structure

struct iomap_dio_ops

Used for Direct-IO. These will call iomap_dio_write().

• struct iomap_dio_ops xfs_dio_write_ops->end_io() - iomap: add a filesystem hook for direct I/O bio submission

• struct iomap_dio_ops xfs_dio_write_ops->submit_io() - xfs: split the iomap ops for buffered vs direct writes

struct iomap_writeback_ops

• struct iomap_writeback_ops xfs_writeback_ops - xfs: support CoW in fsdax mode

Converting filesystems from buffer-head to iomap guide

These are generic guidelines on converting a filesystem over to iomap from buffer-heads.

One op at at time

You may try to convert a filesystem IO operation at time, for instance this order reflects the order in which XFS started converting over to iomap:

• xattr
• seek
• direct writes
• buffered writes
• read
• DAX writes

Defining a simple filesystem

A simple filesystem is perhaps the easiest to convert over to iomap, a simple filesystem is one which:

• does not use fsverify, fscrypt, compression
• has no direct overwrites
• has no Copy on Write support (reflinks)

Converting a simple filesystem to iomap

Simple filesystems should covert to IOMAP directly and avoid buffer heads directly, ie, don't use IOMAP_F_BUFFER_HEAD.

Converting shared filesystem features

fscrupt, fsverity, compression needs to be converted first to iomap if a fs uses it as iomap supports no permutations (XXX: clarify on this)

Converting complex filesystems

If your filesystem does not fit the simple description above the general recommendation is to port to iomap with IOMAP_F_BUFFER_HEAD in one kernel release to verify you no bugs with, locking, writeback and general use of your new struct iomap_ops.

When to set iomap on srcmap or dstmap

The struct iomap is required to be set on iomap_begin(), if its a CoW path also set srcmap when used with iomap_begin().

This perhaps should be redesigned in the future depending on read / write requirements and it may take time to get this right.

Removal of IOMAP_F_BUFFER_HEAD

IOMAP_F_BUFFER_HEAD won't be removed until we have all filesystem fully converted away from buffer-heads, and this could be never.

Testing Direct IO

Other than fstests you can use LTP's dio, however this tests is limited as it does not test stale data.

./runltp -f dio -d /mnt1/scratch/tmp/

Known issues and future improvements

Other than lack of documetnation there are some known issues and limitatiosn with iomap at this time. We try to itemize them here:

• write amplification on IOMAP when bs < ps

  • iomap needs improvements for large folios for dirty bitmap tracking

Q&A

• Why does btrfs only have a few IOMAP calls:
  • btrfs manages page cache folios for buffered IO?

References

• Presentation on iomap evolution

• LWN review of deprecating buffer-heads]